Two projects are involved: 1) Incubation of living tissues such as liver slices or reticulocyte suspensions with highly radioactive amino acids permits the determination of the frequency with which the radioactive amino acid (i.e. valine) substitutes for a closely related amino acid (i.e. isoleucine) during normal protein biosynthesis. For instance we find in rabbit hemoglobin synthesis that valine substitutes for isoleucine about one time in 3000. We are extending these studies to liver synthesis of ferritin in an effort to relate error frequency to age. 2) We are doing classic enzyme kinetics and rapid stopped flow kinetics on the several reactions involved in the synthesis of aminoacyl-tRNA. We believe that the usual stepwise formulation for this reaction is not generally correct. BIBLIOGRAPHIC REFERENCES: T. Lovgren, J. Heinonen and R.B. Loftfield, "Reactivity of Enzyme-Bound Isoleucyl Adenylate" in J. Biol. Chem. expected May, 1975. R.B. Loftfield and A. Pastuszyn, "Aminoacylation of tRNA as the Growth-Limiting Step in Putrescin Deficient E. coli," Fed. Proc. 34, 501 (1975).